Microelectromechanical (MEM) devices have applications for forming many and varied types of microsensors and microactuators. The monolithic integration of MEM devices with electronic circuitry offers the possibility for increasing performance (e.g. forming "smart sensors" having driving, control, and signal processing circuitry formed therewith on a substrate, also termed a wafer or chip) and reliability as well as significantly reducing size and cost. Furthermore, the sensitivity of many types of microsensors (e.g. accelerometers) can be improved by a reduced noise level provided by on-chip circuitry.
U.S. Pat. No. 5,326,726 to Tsang et al discloses an interleaved or merged process approach for fabricating a monolithic chip containing integrated circuitry interconnected to a microstructure (i.e. a MEM device). The approach of Tsang et al requires that the separate steps for forming the MEM device and the integrated circuit be interleaved for compatibility, with the electronic circuitry being formed at least in part prior to the MEM device, and electrical interconnections between the circuitry and the MEM device being formed thereafter. While Tsang et al use some essentially standard process steps, other process steps must be modified due to conflicting requirements between the circuitry and MEM devices.
These modified process steps are primarily dictated by thermal cycles and topography during processing which are largely responsible for determining a strategy for developing a merged or interleaved approach for integrating MEM devices with electronic circuitry. As an example, "islands" of severe topography can be formed by MEM devices extending upward several microns above the substrate, requiring modifications to photolithography and etching processes for forming electrical interconnections between the MEM devices and circuitry. Such modification of process steps to the extent that it deviates from standard processing steps is disadvantageous and costly in requiring that the modified process steps be adapted to a particular type of MEM device, and altered for fabrication of other types of MEM devices. The development of non-standard process steps for forming electronic circuitry that are dictated by requirements of a particular MEM device is disadvantageous in requiring a lengthy period of time for process modification or re-engineering, thereby preventing rapid prototyping of different MEM technologies or MEM development work. Furthermore, since process steps for forming electronic circuitry (e.g. comprising CMOS transistors) are well established and standardized, any modification of the process steps can significantly decrease the circuit performance and the overall process yield.
What is needed is a method for integrating MEM devices with electronic circuitry that substantially separates the process steps for fabricating the MEM devices from the process steps for fabricating the electronic circuitry, thereby allowing the use of standard process steps as known to the art, especially for fabricating the electronic circuitry.
Heretofore, such a separation of steps for fabricating MEM devices and steps for fabricating electronic circuitry has been based on a method of fabricating the electronic circuitry prior to fabrication of the MEM devices in a circuitry-first approach. This approach has been primarily motivated by concerns about contamination and a rough topography that is generally thought to be inevitable if the MEM devices were fabricated first. A rough topography places severe demands on subsequent lithography and etching processes for forming the electronic circuitry. The use of a circuitry-first approach, however, is disadvantageous in requiring deviations from standard processing steps (i.e. process modifications), especially in requiring the use of tungsten instead of aluminum for the interconnect metallization to withstand a high-temperature annealing step required to at least partially relieve stress in polysilicon elements (e.g. cantilevered beams) of MEM devices. However, the use of tungsten as an interconnect metallization is not altogether satisfactory, resulting in additional problems including a high contact resistance and hillock formation that can lead to inadequate protection of the electronic circuitry during release of the MEM devices. Additional problems known to occur with this circuitry-first approach include an undesirable formation of tungsten silicides, and poor adhesion of the tungsten interconnect metallization.
An advantage of the method of the present invention is that microelectromechanical (MEM) devices can be integrated with electronic circuitry on a substrate while using standard process steps with little if any modification for fabricating the electronic circuitry, including the use of an aluminum interconnect metallization in preferred embodiments of the present invention.
Another advantage of the present invention is that one or more MEM devices can be fabricated prior to fabrication of electronic circuitry, with the MEM devices being encapsulated to prevent contamination of a device surface of the substrate.
A further advantage of the present invention is that the substrate can be planarized prior to formation of the electronic circuitry thereby providing a substantially smooth and planar surface topography for subsequent process steps for fabricating the electronic circuitry.
Still another advantage of the present invention is that the encapsulated MEM devices can be annealed under temperature and time conditions sufficient to relieve strain in elements of the MEM devices prior to formation of the electronic circuitry including the interconnect metallization.
Yet another advantage is that by providing one or more encapsulated MEM devices formed within a cavity below a device surface of a planarized substrate, the substrate can be handled and processed thereafter using substantially standard process steps with little if any modification for forming the electronic circuitry (including the interconnect metallization).
Another advantage of the present invention is that the electronic circuitry can be protected during an etching step for releasing the encapsulated MEM device by covering the electronic circuitry with a layer of deposited tungsten that can be removed without damaging the underlying electronic circuitry after the MEM device is released.
Yet another advantage of the method of the present invention is that packaging of both MEM and integrated devices (i.e. MEM devices integrated with electronic circuitry) can be simplified and cost reduced by forming one or more of the MEM devices below a device surface of a substrate with an overlying layer forming at least a part of an enclosure for packaging the devices.
These and other advantages of the method of the present invention will become evident to those skilled in the art.